1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to the art of reducing the power consumption of a liquid crystal display.
2. Description of the Related Art
One conventional active matrix liquid crystal display in which active elements are connected to respective pixels for energizing a liquid crystal is known from "ELECTRONIC DISPLAY" edited by Shouichi Matsumoto, p. 66-67, published by Ohm-sha. A liquid crystal display described in this literature is illustrated in FIG. 1 of the accompanying drawings.
The liquid crystal display shown in FIG. 1 comprises a matrix of data bus lines 2 and gate bus lines 3 (l-m) which cross at right angles to each other, and a plurality of thin-film transistors (TFTs) 4 disposed near regions where the data bus lines 2 and the gate bus lines 3 cross each other, for controlling currents flowing from the data bus lines 2 to pixel electrodes 1 with signals supplied from the gate bus lines 3. FIG. 1 shows a section of the liquid crystal display which corresponds to one of the data bus lines 2.
Signals, described below, are supplied to the liquid crystal display to energize same. FIGS. 2A and 2B are timing charts of such signals. When the gate bus lines 3 are successively selected and turned on at given periods, as shown in FIG. 2A, a TFT 4 connected to the selected gate bus line 3 is turned on, writing a signal from the data bus line 2, which is being energized by an alternating-current signal with respect to a confronting electrode voltage, into the corresponding pixel electrode 1 (see FIG. 2B). This operation is repeated from the first gate bus line 3 to the mth gate bus line 3 for thereby completing the display of one frame. When one cycle of scanning up to the mth gate bus line 3 is finished, another cycle of scanning is started from the first gate bus line 3.
In the conventional liquid crystal display, since each time a gate bus line 3 is turned on a signal voltage is applied to the data bus line 2, the total quantity of electric charges supplied to and removed, for example, from one data bus line during a period of one frame when a solid black image is displayed on the entire display panel is expressed by: EQU Q.sub.1 .times.m
where Q.sub.1 represents the amount of electric charges required for one data bus line 2 and m represents the number of gate bus lines.
Since the entire quantity of electric charges for the entire display panel is calculated by multiplying the above total quantity of electric charges by the number of data bus lines, the calculated entire quantity of electric charges is large enough to increase the power consumption of the liquid crystal display. Because the above charges are proportional to the area of the data bus lines, the power consumption increases as the size of the data bus lines increases even if the number of pixels used remains the same. Consequently, even if the number of pixels used remains the same, as the display panel size increases, the electric power required to energize the liquid crystal capacity increases though the electric energy consumed by a signal processing system does not increase.
Therefore, if the conventional liquid crystal display is incorporated in a battery-powered unit such as a personal digital assistant, the period of time for which the battery-powered unit can be used without being recharged is reduced.